cmd/compile: drop "buildcfg" from no instrument packages

Package buildcfg was added to this list by CL 403851, but package
buildcfg does not exist.

This was probably intended to refer to internal/buildcfg, but
internal/buildcfg is only used by the compiler so it is not clear why it
couldn't be instrumented.

For #44853.

Change-Id: Iad2517358be79c3eabf240376156bcff0c4bcefc
Reviewed-on: https://go-review.googlesource.com/c/go/+/414516
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>

src/cmd/compile/internal/base/base.go

@@ -70,7 +70,6 @@ var NoInstrumentPkgs = []string{
 	"runtime/msan",
 	"runtime/asan",
 	"internal/cpu",
-	"buildcfg",
 }
 
 // Don't insert racefuncenter/racefuncexit into the following packages.